1. Field of the Invention
The present invention relates to a Hall device for detecting a magnetic field, and particularly, to a Hall device that is reliable and accurate.
2. Description of the Prior Art
A Hall device is a magnetic transducer using Hall effect, resulting from the Lorentz force on a moving charge carrier in an applied magnetic field, to pick up an electromotive force to be generated orthogonally to the outer product of an input current vector and a flux vector. The Hall device is formed on the surface of a pellet so that the direction of an input current is perpendicular to the direction of an output voltage.
FIGS. 1 and 2 show each an example of a semiconductor Hall device according to a prior art. Each of the Hall devices consists of a pellet 1 made of a semi-insulating GaAs substrate, a cross-shaped active region 2 formed on the pellet 1, an insulation film 4 made of silicon oxide to cover the surface of the substrate, a pair of input terminals 3, and a pair of output terminals 5. The input and output terminals 3 and 5 are electrically connected to the active region 2 through contact windows formed through the insulation film 4. The input and output metal electrode terminals 3 and 5 are used as bonding pads. Each of the Hall devices is formed from a semi-insulating GaAs wafer whose surface is in a (100) plane so that the side face of the pellet is in the (100) plane or a (110) plane. The orientation of the side face of a pellet is determined according to how the pellet is diced from a wafer. When the pellet is diced by scribing, the side face thereof is in the (100) plane because the direction of dicing is aligned with the direction of cleavage. When the pellet is diced with a blade, the side face thereof is in the (110) plane, to prevent the chipping of the wafer. In FIG. 1, the side face of the pellet 1 is in the (100) plane, and a current path is arranged at an angle of 45 degrees with respect to the edges of the pellet 1. In FIG. 2, a current path is arranged in parallel with the edges of the pellet 1. The input and output terminals 3 and 5 are usually arranged at the corners of the pellet 1, to improve the area efficiency of the pellet 1. Table 1 shows a combination of the orientations and directions of the faces and input/output paths of a Hall device according to the prior art.
TABLE 1 ______________________________________ Side face Active layer orientation Direction of Direction of orientation of pellet input current output voltage ______________________________________ {100} {100} [100] [100] {100} {100} [110] [110] {100} {110} [100] [100] {100} {110} [110] [110] ______________________________________
The pellet 1 is mounted on a mold substrate, is bonded to wires, and is packaged with mold peripheral devices into a product.
An electromotive force measured between the output terminals of the Hall device is theoretically zero if there is no magnetic field. In practice, however, the electromotive force will not be zeroed due to minute asymmetry in the Hall device, or a fluctuation in the specific resistance of the active region of the device. Such a non-zero output is called an offset output V.sub.HO.
The reason why a fluctuation in the specific resistance of the active region of the Hall device causes the non-zero offset output V.sub.HO is because the specific resistance of the active region is closely related to the piezoelectric characteristic of the semiconductor crystal of the Hall device. The offset output V.sub.HO will vary if external stress is applied to locally change the specific resistivity. The reason why minute asymmetry in the Hall device causes the non-zero offset output V.sub.HO will be explained with reference to FIG. 4. The Hall device of FIG. 4 has a GaAs semiconductor substrate. An input bias voltage is applied in a [100] direction to the active region of the substrate. A voltmeter 7 measures a potential change in, for example, a [01O] direction, to detect the strength of a magnetic field that vertically traverses the Hall device. If a stress is applied at an angle of 45 degrees in an intermediate direction between the [100] and [01O] directions as shown in FIG. 4, the offset output V.sub.HO due to asymmetry in the device is zero because the stress is symmetrical with respect to these directions. If the direction of the stress forms an angle of .theta. with respect to the 45-degree direction as indicated with a dot-and-dash line in FIG. 4, the offset output V.sub.HO produced between the output terminals of the Hall device with no magnetic field is as follows: ##EQU1## where
.rho..sub.1 : piezoresistance coefficient in the longitudinal direction of the current
.rho..sub.t : piezoresistance coefficient in the transversal direction of the current
Y: Young's modulus of the active layer
S: strain of the active layer
Vs: voltage applied to the device
When there is a magnetic field, the output V.sub.M of the Hall device is as follows: EQU V.sub.M =V.sub.H -V.sub.HO ( 2)
When there is no magnetic field, the output V.sub.M of the Hall device is as follows: EQU V.sub.M =V.sub.HO ( 3)
where V.sub.H is a change in the electromotive force between the output terminals of the Hall device due to the magnetic field.
When leads are soldered to the Hall device, thermal stress is applied to the device depending on the thermal-expansion coefficient thereof. The thermal stress is also caused in a curing process, to change the offset output V.sub.HO. This results in changing the output V.sub.M of the Hall device. FIG. 3 shows a result of a solder step stress test to measure a change in the offset output V.sub.HO of a Hall device during a lead soldering process. For the test, 20 samples, for example, of Hall devices are picked up. The offset output of each of the samples is measured at the room temperature. This offset output is an initial offset output V.sub.HOO. The samples are dipped in a solder bath of 260.degree. C. for 10 seconds and are cooled to the room temperature. Then, the offset output of each of the samples is measured. This offset output is V.sub.HO (260.degree. C.). A difference between the measured offset outputs is calculated as .DELTA. V.sub.HO (260.degree. C.)=V.sub.HO (260.degree. C.)-V.sub.HOO. The average and variance of the differences of the samples are calculated. These processes are repeated by increasing the temperature of the solder bath at intervals of 20.degree. C. Namely, the test is repeated at 280.degree., 300.degree., 320.degree., and 340.degree. C. At the same time, differences are calculated as .DELTA. V.sub.HO (280.degree. C.)=V.sub.HO (280.degree. C.)-V.sub.HOO, .DELTA. V.sub.HO (300.degree. C.)=V.sub.HO (300.degree. C.) -V.sub.HOO, and the like. As shown in FIG. 3, the differences .DELTA. V.sub.HO (260.degree. C.), .DELTA. V.sub.HO (280.degree. C.), etc., in the offset output V.sub.HO of the conventional Hall device range from .+-.1 mV to .+-.2 mV. Namely, the reliability of the conventional Hall devices is insufficient to precisely measure a magnetic field.
To deal with this problem, a user must cancel, when accurately measuring a magnetic field, the offset output V.sub.HO of a Hall device by adjusting circuit constants of the device when assembling the device in a system. The system, however, will be incorrect if the offset output V.sub.HO varies thereafter. Namely, the conventional Hall devices may roughly detect, for example, the rotational position of a rotor of a brushless motor but are unreliable and inaccurate for measuring a magnetic field in current transformers and wattmeters, or for detecting the rotational position of devices installed in a car.